Actuators are used for many purposes in controlling machinery. For example, in sawmills actuators are used for bar turners and carriage loading arms to control the position of a log or cant on a carriage which will then transport the log to the headrig. Such actuators are generally controlled by a control valve which will provide movement to the piston in the actuator, the piston regulating the flow of air from the actuator for controlling the machinery.
Bar turners will generally include pull up and pull down air cylinders which are used to position the log on the carriage. The cylinders are operated by air passing through the actuator. By controlling the position of a piston within the actuator, the operator can control the bar turner which, in turn, will control the position of the log. The control of a log position is important for optimum lumber return during the cutting of the log.
The position of the piston in the actuator is generally controlled by a control valve, the control valve having a spool which moves and which is pneumatically connected to the actuator to control its piston. In one known design, a control system comprises a control valve having a spool which is movable responsive to a piston of a hydraulic cylinder which is moveable by a hydraulically operated pilot. The spool of the cylinder is mechanically connected to the spool of the control valve so that when the piston of the cylinder moves, the spool of the control valve will move with the result that the amount and pressure of air leaving the actuator can be controlled.
The control valve, in turn, is generally controlled by a slave cylinder or valve. The slave cylinder or valve is intended, by exerting influence over the position of the spool in the control valve, to precisely control the operation of the actuator. Thus, the operator, by operating the slave cylinder or valve, can exert control over the actuator and the machinery to which the actuator is connected.
Heretofore, the preferred medium for use in the slave cylinder, the control valve and the actuator has been steam. Steam does not have substantial compressibility and its pressure is relatively high. As such, the operator could develop a good "feel" for the operation of the actuator by operating the slave cylinder. Good feel to the operator is important to precisely control the position of the log or cant on the carriage, since the maximization of lumber from a log or cant is highly desirable in a sawmill operation and this maximization is, in turn, obtained at least in part by the position of the log on the carriage.
Steam, however, has disadvantages. Most importantly, it is expensive to generate and, in some sawmill operations because of their location or otherwise, steam may be difficult to generate, cost considerations aside.
One solution proposed to replace the steam system has been a system such as that aforementioned where the actuator was mechanically controlled by the control valve and the control valve, in turn, was controlled by hydraulic fluid leaving the pilot valve. The mechanical linkage used, however, was disadvantageous since a "lag" occurred between the time that the pilot was actuated and the time the actuator made its move responsive to the move of the spool in the pilot. In addition, the mechanical linkage inherently had tolerances which were relatively large. This resulted in the piston of the actuator being only generally positioned with the result that the output of the actuator piston could not be precisely controlled. This, of course, is disadvantageous as set forth above.
A further known technique was to use a control valve which controlled the actuator with air. This solution was found to be deficient because the feel of the device was relatively poor due to the compressibility of air and the fact that the actuator could not be precisely positioned.